ACTIVE GALACTIC NUCLEI: BLAZARS (original) (raw)
Related papers
HESS J1943+213: An Extreme Blazar Shining through the Galactic Plane
The Astrophysical Journal, 2018
HESS J1943+213 is a very high energy (VHE; >100 GeV) γ-ray source in the direction of the Galactic plane. Studies exploring the classification of the source are converging toward its identification as an extreme synchrotron BL Lac object. Here we present 38 hr of VERITAS observations of HESS J1943+213 taken over 2 yr. The source is detected with a significance of ∼20 standard deviations, showing a remarkably stable flux and spectrum in VHE γ-rays. Multifrequency Very Long Baseline Array (VLBA) observations of the source confirm the extended, jetlike structure previously found in the 1.6 GHz band with the European VLBI Network and detect this component in the 4.6 and 7.3 GHz bands. The radio spectral indices of the core and the jet and the level of polarization derived from the VLBA observations are in a range typical for blazars. Data from VERITAS, Fermi-LAT, Swift-XRT, the FLWO 48″ telescope, and archival infrared and hard X-ray observations are used to construct and model the spectral energy distribution (SED) of the source with a synchrotron self-Compton model. The well-measured γ-ray peak of the SED with VERITAS and Fermi-LAT provides constraining upper limits on the source redshift. Possible contribution of secondary γ-rays from ultra-high-energy cosmic-ray-initiated electromagnetic cascades to the γ-ray emission is explored, finding that only a segment of the VHE spectrum can be accommodated with this process. A variability search is performed across X-ray and γ-ray bands. No statistically significant flux or spectral variability is detected.
Characterizing X-ray variability of blazars
2008
In this review, I will discuss how to characterize synchrotron X-ray variability of TeV blazars by using the observed/simulated light curves. Apparently, temporal studies provide independent and complementary information to the spectral studies, but surprisingly little attention has been paid especially for the blazar study. Only exception is a classical argument for presence of "time lag", which may (or may not) reflect the diffrence of synchrotron cooling timescale. Also very recently, it was suggested that the X-ray variability of TeV blazars indicates a strong red-noise, compared to a fractal, flickering-noise of Seyfert galaxies. Various temporal techniques are proposed in literature, e.g., the power spectrum density (PSD), the structure function (SF), and the discrete correlation function (DCF) and other analysis tools, but special care must be taken if the data are not well sampled and observation is relatively short compared to a characteristic timescale of the system. Also, the situation is being more complicated for low-Earth orbit satellites, e.g., ASCA, RXT E and BeppoSAX, since the light curve inevitably contains "periodic gap" due to the Earth occultation (every ≃ 6ksec). I will present detailed approaches to see how the "gap" and the "finite length" of the data affects the results of temporal analysis, and to what extent we can believe in our results. Finally, I will briefly comment on the high-sensitivity X-ray observations with MAXI, that may shed new light on the forthcoming GLAST era.
X-ray spectra, light curves and SEDs of blazars frequently observed by Swift
Monthly Notices of the Royal Astronomical Society, 2021
Blazars research is one of the hot topics of contemporary extragalactic astrophysics. That is because these sources are the most abundant type of extragalactic-ray sources and are suspected to play a central role in multi-messenger astrophysics. We have used swift_xrtproc, a tool to carry out an accurate spectral and photometric analysis of the Swift-XRT data of all blazars observed by Swift at least 50 times between December 2004 and the end of 2020. We present a database of X-ray spectra, best-fit parameter values, count-rates and flux estimations in several energy bands of over 31,000 X-ray observations and single snapshots of 65 blazars. The results of the X-ray analysis have been combined with other multi-frequency archival data to assemble the broadband Spectral Energy Distributions (SEDs) and the long-term lightcurves of all sources in the sample. Our study shows that large X-ray luminosity variability on different timescales is present in all objects. Spectral changes are also frequently observed with a "harder-when-brighter" or "softer-when-brighter" behaviour depending on the SED type of the blazars. The peak energy of the synchrotron component (peak) in the SED of HBL blazars, estimated from the log-parabolic shape of their X-ray spectra, also exhibits very large changes in the same source, spanning a range of over two orders of magnitude in Mrk421 and Mrk501, the objects with the best data sets in our sample.
Testing the blazar spectral sequence: X-ray-selected blazars
Monthly Notices of the Royal Astronomical Society, 2008
We present simultaneous optical and X-ray data from Swift for a sample of radio-loud flat spectrum quasars selected from the Einstein Medium Sensitivity Survey (EMSS). We present also a complete analysis of Swift and INTEGRAL data on 4 blazars recently discussed as possibly challenging the trends of the hypothesised "blazar spectral sequence". The SEDs of all these objects are modelled in terms of a general theoretical scheme, applicable to all blazars, leading to an estimate of the jets' physical parameters. Our results show that, in the case of the EMSS broad line blazars, X-ray selection does not lead to find sources with synchrotron peaks in the UV/X-ray range, as was the case for X-ray selected BL Lacs. Instead, for a wide range of radio powers all the sources with broad emission lines show similar SEDs, with synchrotron components peaking below the optical/UV range. The SED models suggest that the associated IC emission should peak below the GeV range, but could be detectable in some cases by the Fermi Gamma-Ray Space Telescope. Of the remaining 4 "anomalous" blazars, two highly luminous sources with broad lines, claimed to possibly emit synchrotron X-rays, are shown to be better described with IC models for their X-ray emission. For one source with weak emission lines (a BL Lac object) a synchrotron peak in the soft X-ray range is confirmed, while for the fourth source, exhibiting narrow emission lines typical of NLSy1s, no evidence of X-ray emission from a relativistic jet is found. We reexamine the standing and interpretation of the original "blazar spectral sequence" and suggest that the photon ambient, in which the particle acceleration and emission occur, is likely the main factor determining the shape of the blazar SED. A connection between SED shape and jet power/luminosity can however result through the link between the mass and accretion rate of the central black hole and the radiative efficiency of the resulting accretion flow, thus involving at least two parameters.
Relative Timing of Variability of Blazars at X-Ray and Lower Frequencies
X-Ray Timing 2003, 2003
The rich X-ray light curves of blazars obtained with RXTE allow meaningful correlation analyses with longer wavelengths. This reveals strong connections of variations across the electromagnetic spectrum. In 3C 279, PKS 1510 089, and BL Lac, the characteristics of the X-ray variability change along with the projected direction of the compact jet. Outbursts in the radio, IR, or optical often precede flares at high energies. A period of pronounced variability in BL Lac in late 2000 occurs at both optical and X-ray frequencies, with the X-ray spectral index steepening. A superluminal radio knot is ejected during this event. The implication of our monitoring results is that the IR to X-ray (as well as γ-ray) emission is cospatial with the compact radio jet, most likely occurring in the superluminal knots. In the radio galaxy 3C 120, in which the X-rays probably come mainly from a hot accretion-disk corona, the appearance of superluminal radio knots follows (by 4 weeks) dips in the X-ray emission, as in microquasars but on longer timescales. The delay implies that the core of the radio jet, as seen in mm-wave VLBA images, lies at least 0.4 pc from the central engine, consistent with models in which the jet flow accelerates far from the black hole. The quasar 3C 273 may be an interesting hybrid case in which contributions to the X-ray emission may come from both the jet and corona. The power spectral density has a low-frequency break that, in analogy with black-hole binary systems, implies a mass of the central black hole of 3 6 ¢ 10 8 M ¬ , similar to that obtained by reverberation mapping of emission-line variability.
Characterizing X-ray Variability of TeV Blazars
Pos, 2008
In this review, I will discuss how to characterize synchrotron X-ray variability of TeV blazars by using the observed/simulated light curves. Apparently, temporal studies provide independent and complementary information to the spectral studies, but surprisingly little attention has been paid especially for the blazar study. Only exception is a classical argument for presence of "time lag", which may (or may not) reflect the diffrence of synchrotron cooling timescale. Also very recently, it was suggested that the X-ray variability of TeV blazars indicates a strong red-noise, compared to a fractal, flickering-noise of Seyfert galaxies. Various temporal techniques are proposed in literature, e.g., the power spectrum density (PSD), the structure function (SF), and the discrete correlation function (DCF) and other analysis tools, but special care must be taken if the data are not well sampled and observation is relatively short compared to a characteristic timescale of the system. Also, the situation is being more complicated for low-Earth orbit satellites, e.g., ASCA, RXTE and BeppoSAX, since the light curve inevitably contains "periodic gap" due to the Earth occultation (every ~6ksec). I will present detailed approaches to see how the "gap" and the "finite length" of the data affects the results of temporal analysis, and to what extent we can believe in our results. Finally, I will briefly comment on the high-sensitivity X-ray observations with MAXI, that may shed new light on the forthcoming GLAST era.
Astronomy and Astrophysics, 2022
Context. The multi-wavelength (MWL) properties of some TeV-detected blazars are still poorly understood. By studying the MWL properties of the BL Lacertae source 1ES 0033+595, we make further progress in our understanding of the instable physical processes responsible for particle acceleration to ultra-relativitic energies and the observed emission properties. Aims. This paper presents the results of a detailed X-ray timing and spectral analysis of the source 1ES 0033+595, allowing us to draw conclusions about the physical mechanisms responsible for particle acceleration and the generation of X-ray emission. We also examined the long-term MWL behaviour of the source and interband cross-correlations. Methods. Our study focused on the observations performed with the X-Ray Telescope on board the Neil Gehrels Swift Observatory (Swift-XRT) in the period 2005-2022. The MWL data sets were obtained with several space and ground-based instruments. We analysed the MWL flaring behaviour during the entire period, as well as in particular subperiods selected on the basis of the X-ray flaring activity of the target; constructed histograms characterising the distributions of the different spectral parameters and MWL fluxes; and checked cross-correlations between these parameters and/or MWL fluxes. The obtained results are compared with the predictions of the various theoretical studies and simulations presented by the different authors to date. Results. The source showed extreme spectral properties with dominance of high spectral curvature, frequent occurrence of very and extremely hard photon indices, and the presence of a synchrotron spectral energy distribution (SED) peak in the hard X-ray range (sometimes at energies beyond 10 keV). These properties reveal the importance of first-order Fermi acceleration with very low initial particle energy distribution, along with the coexistence of stochastic acceleration and hadronic processes. The source was characterised by very uneven and erratic flaring activity in diverse epochs: the period of strong flares (2013-2016) was preceded by a moderate variability and followed by a gradual long-term decrease in MWL flaring activity. We detected a number of instances of intraday 0.3-10 keV variability, which were sometimes observed within a few hundred seconds and explained by the interaction between the relativistic shock front and jet inhomogeneities with strong magnetic fields. The X-ray and γ-ray fluxes showed a lognormal distribution, which hints at the imprint of accretion disc instabilities on the blazar jet.
Long-term optical and gamma−\gamma-gamma−ray variability of the blazar PKS~1222+216
Cornell University - arXiv, 2022
The γ−ray emission from flat-spectrum radio quasars (FSRQs) is thought to be dominated by the inverse Compton scattering of the external sources of photon fields, e.g., accretion disk, broad-line region (BLR), and torus. FSRQs show strong optical emission lines and hence can be a useful probe of the variability in BLR output, which is the reprocessed disk emission. We study the connection between the optical continuum, Hγ line, and γ−ray emissions from the FSRQ PKS 1222+216, using long-term (∼2011-2018) optical spectroscopic data from Steward Observatory and γ−ray observations from Fermi-LAT. We measured the continuum (F C,opt) and Hγ (F Hγ) fluxes by performing a systematic analysis of the 6029-6452Å optical spectra. We observed stronger variability in F C,opt than F Hγ , an inverse correlation between Hγ equivalent width and F C,opt , and a redder-when-brighter trend. Using discrete cross-correlation analysis, we found a positive correlation (DCF∼0.5) between F γ−ray>100MeV and F C,opt (6024-6092Å) light curves with time-lag consistent with zero at 2σ level. We found no correlation between F γ−ray>100MeV and F Hγ light curves, probably dismissing the disk contribution to the optical and γ-ray variability. The observed strong variability in the Fermi-LAT flux and F γ−ray>100MeV − F C,opt correlation could be due to the changes in the particle acceleration at various epochs. We derived the optical-to-γ-ray spectral energy distributions (SEDs) during the γ-ray flaring and quiescent epochs that show a dominant disk component with no variability. Our study suggests that the γ-ray emission zone is likely located at the edge of the BLR or in the radiation field of the torus.